Ultrasound is one known method for analyzing microbial populations such as are grown in fermenters. Such a technique is described, for example, in Applied and Environmental Microbiology, Vol. 42, p. 632-637 (1981) and in J. of the Accoustical Society of America, Vol. 31, pp. 185-189 (1959). The process is based upon the fact that the bulk properties of the dispersion change with cell growth. This is in turn affects the rate of attenuation of the ultrasound waves.
However, this is considerable difficulty in making such measurements within the fermenter itself. A chief difficulty is that the fermentation liquid of necessity is aerated, and the presence of bubbles (from 10 to 15% of the volume) grossly distorts the ultrasound attenuation so as to suggest an apparent change to the overall density. (As used herein, "aerated" refers to the presence of gas bubbles of any suitable composition, such as air in the case of aerobic microbes, and ammonia in the case of anaerobic microbes.) One solution to this would be to remove sample from the fermenter to an off-line location. However, this requires extensive equipment and delay, so that the measurement being taken may not be an accurate measurement of instantaneous conditions as they exist in the fermenter. The opposite solution of measuring within the fermenter has not seemed desirable in the past due to the omnipresent bubbles. That is, if a portion of the fermenter volume is debubbled prior to making the measurement, it is sure to become reaerated almost instantly, so that a bubble-free measurement did not appear to be possible.
Yet another approach has been to attempt to make the ultrasound measurement in a portion of the fermenter that is free of the bubbles. Such an approach is suggested in Japanese Kokai No. 61/74573. The difficulty with this method is that in many cases, the aeration occurs throughout the fermenter, so that no place is completely free of bubbles. It is unrealistic therefore to attempt to identify the portion of the fermenter that is free of bubbles.
On-line sensing of fermentation baths has been described, using a debubbling means, in Biotechnology and Bioengineering Symp., No. 9, page 103-116 (1979). However, debubbling is done by a passive, complex density separation rather than an active debubbling technique. Thus, there is no forcing of the bubbles away from the sensor portion. In addition, the liquid follows a long, tortuous path that encourages deposits to form. Such deposits render cleaning difficult, if not impossible. Finally, because density separation is used for the bubbles, it is not possible to entirely submerge the device (shown in FIG. 7 of the article). Instead, the top portion must project above the liquid to allow the bubbles to communicate with the atmosphere above the liquid. At the same time, however, the liquid inlet of the apparatus must be below the liquid level, so that the apparatus becomes very sensitive to that liquid level.